I agree with Kevin. I"ve never seen a reliable sub you can work from under the mount. Your only option is to look to escape the position. Which is actually pretty easy on someone with little or no ground training, alot harder against an experienced BJJer. But if you're fighting an experienced BJJer you're in alot of trouble where ever you end up.
Glen maybe you're confusing mount with guard?

"When your only tool is a hammer every problem starts to look like a nail"